//对象转查询字符串
function objToSearchString(obj) {
    var str = ''
    for (var key in obj) {
        str += `${key}=${obj[key]}&`;
    }
    return str.slice(0, str.length - 1);
}

//封装ajax函数
function ajax(method, url, params, cb) {
    //1.创建请求对象
    var xhr = new XMLHttpRequest()

    //2.配置请求对象
    xhr.open(method, method == 'get' ? url + "?" + objToSearchString(params) : url);

    //3.设置回调函数
    xhr.onreadystatechange = function () {
        if (xhr.readyState == 4) {
            cb(JSON.parse(xhr.responseText))
        }
    }

    //4.发送请求
    //先对post请求设置 content-type 请求头
    xhr.setRequestHeader('content-type', 'application/x-www-form-urlencoded');
    xhr.send(method == 'get' ? null : objToSearchString(params));
}


